The rate at which solar energy is stored in green plants to be used as food is called:
- (a)Primary productivity
- (b)Secondary productivity
- (c)Tertiary productivity
- (d)Respiration
Correct — A, Primary productivity.
Productivity in ecology is a rate: organic matter created per unit area per unit time. The producer step of that rate — sunlight fixed by green plants into food — carries the label Primary productivity.
The stem gives three cues that point at the producer step. It says rate, it says green plants, and it says the product is food. Steps further along the food chain fail the second cue, since the organisms there are not green plants.
The term splits two ways. Gross primary productivity is the total organic matter fixed during photosynthesis; net primary productivity is what survives the plant's own respiration, and that remainder is what heterotrophs feed on.
Carry away that productivity is a rate of production, not the amount of biomass standing there at a moment — the amount present at an instant is the standing crop.
- (b)Secondary productivity — Secondary productivity measures the consumer step, not the producer step. A herbivore or carnivore builds new tissue out of food it has eaten, and the energy in that food was fixed earlier by plants — so no solar energy is being stored here.
This option is the right answer to the rate of formation of new organic matter by consumers, the heterotroph half of the productivity pair.
- (c)Tertiary productivity — NCERT defines productivity only at two levels, primary (producers) and secondary (consumers); there is no third tier in that scheme, and the stem's producer cues fix the answer at primary regardless.
Energy reaching a carnivore that eats other carnivores was fixed by green plants several steps earlier, so the sunlight-storing step keeps the name primary however long the chain runs.
'Tertiary' is the right label for the trophic position of a carnivore that feeds on other carnivores — a tertiary consumer, the third consumer level above the producer.
- (d)Respiration — Respiration runs the opposite way to the stem. It oxidises food that has already been made, releasing the energy for the cell's work and giving out carbon dioxide and water — it spends the store rather than laying it down.
It is the right answer to the process in which food is oxidised to release energy, carbon dioxide and water. In ecosystem accounting respiration is the quantity subtracted from gross primary productivity to give the net figure.
Productivity in ecology is the rate at which organic matter is created in an ecosystem, expressed as biomass or as energy, per unit area, per unit time.
It is defined at two levels. Producers capture sunlight and build organic matter — primary productivity. Consumers convert what they have eaten into tissue of their own — secondary productivity.
Primary productivity is reported in two forms. The gross figure is everything fixed during photosynthesis; the net figure is the gross minus what the plant respires away, and it is the part left for other organisms.
This term sits at the entrance to the whole energy-flow chapter. How fast an ecosystem fixes sunlight sets the ceiling for the trophic levels above it, since the levels above draw on what the producers have already fixed and lose a share of it at each transfer.
Once that entry rate is understood, the rest of the chapter follows from it — the transfer of a small fraction of energy to each higher level, the shapes of ecological pyramids, and decomposition returning matter to the soil.
- Primary productivity is the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis — the time term is what makes it a rate rather than a quantity.
- It is expressed either as weight, such as grams per square metre, or as energy, such as kilocalories per square metre, per unit time.
- Gross primary productivity is the rate at which producers fix organic matter in photosynthesis, counted before any of it is respired away.
- Net primary productivity equals gross primary productivity minus the respiratory losses of the plants themselves.
- Net primary productivity is the biomass available to heterotrophs, which include herbivores and decomposers.
- Secondary productivity is the rate of formation of new organic matter by consumers.
- Primary productivity varies with the plant species, the environmental conditions, nutrient availability and the photosynthetic capacity of the plants.
- Standing crop is the biomass present at a given moment, a stock, whereas productivity is a rate measured over an interval of time.
The highlighted row is the one the stem describes. The rows under it measure a part of that store, spend it, or count it at an instant.
- Reading 'productivity' as a quantity of biomass; it is a rate, so the term wanted is the one defined per unit area per unit time, not the standing crop.
- Letting the word 'stored' pull the eye to respiration, which releases what has been stored instead of laying it down.
- Taking 'secondary' because the plant will be eaten later; the prefix names who forms the organic matter — producer or consumer — not what happens to it afterwards.
- Hesitating because neither 'gross' nor 'net' appears in the options; the stem names the category, and primary productivity is the umbrella term under which both measures sit.
One form is the bare definition to label, as in this stem, or the reverse — the term is given and its definition has to be picked out.
A second form embeds the idea in statement-based items on ecosystem energetics, where a line about how much sunlight producers capture, or how much passes to the next level, has to be judged correct or incorrect.
Beyond the wording of the item, the relation GPP − R = NPP is worth being able to run in either direction, so that any one of the three follows from the other two.
UPPSC_2025_PRE_GSI_Q52025Same corner of the syllabus — energy entering an ecosystem through green plants and moving up trophic levels. That item works with the quantities of ecosystem energetics, the share of incident sunlight captured by leaves and the share passed to the next level; the UKPSC stem asks instead for the name of the rate at which that capture happens.
UPSC_2014_GS1_Q652014Both hinge on energy being stored by green plants. The UPSC item asks what happens inside the process — free energy converted into potential energy and stored — while the UKPSC item asks for the ecosystem-level term for the rate of that storage. Process description there, vocabulary here.
MPPSC_2018_PRE_PaperI_Q62018The same conversion, cut at a different point. The MPPSC item asks what sunlight becomes inside the plant, that is, the form of energy it is turned into; the UKPSC item takes the conversion as given and asks what its rate is called.
HPSC_2021_PRE_GSII_Q472021Useful against option (d) here. That item lists events and asks which one does not belong to photosynthesis, with the oxidation of carbon to carbon dioxide among the choices — the same photosynthesis-against-respiration boundary the UKPSC options test, but framed as a process question rather than as a definition.
- practice — not a real PYQ
Net primary productivity of a plant community is best expressed as:
- (a)Gross primary productivity plus respiratory losses
- (b)Gross primary productivity minus respiratory losses
- (c)Gross primary productivity plus secondary productivity
- (d)Secondary productivity minus respiratory losses
Answerb — net primary productivity is what remains of the organic matter fixed in photosynthesis after the plants have respired part of it away, so NPP = GPP − R.(a) adds the losses back and returns the gross figure. (c) mixes the producer and consumer accounts, which are measured separately. (d) starts at the consumer level, where the deduction for plant respiration does not belong.
- practice — not a real PYQ
The rate of formation of new organic matter by consumers in an ecosystem is called:
- (a)Gross primary productivity
- (b)Net primary productivity
- (c)Secondary productivity
- (d)Standing crop
Answerc — secondary productivity is defined as the rate of formation of new organic matter by consumers.(a) and (b) both belong to producers: the gross figure before respiration, the net figure after it. (d) standing crop is the biomass present at a given moment, a stock rather than a rate, and it applies to producers as well as consumers.
- practice — not a real PYQ
Which one of the following is a rate rather than an amount present at a given moment?
- (a)Standing crop of a grassland
- (b)Biomass of the herbivore trophic level
- (c)Net primary productivity
- (d)Number of trees in a forest plot
Answerc — net primary productivity is measured as organic matter per unit area per unit time, which makes it a rate.(a), (b) and (d) are all readings taken at one moment: a standing crop, a biomass and a count. Each can change over the seasons, but the figure itself is a stock, not a speed of production.